The invention relates to an apparatus of bending pipes with local heating.
Known widely at present are the methods of heating pipes with local heating of the bending zone, realized with the aid of devices whose operating element constitutes a bending head which is rotatable in the bending plane, and which clamps one end of the pipe to be bent.
The most general practice is to place the pipe on the bed of the device, moving it at once to the stop in the bending head, and to clamp it there before the beginning of bending, i.e. before applying the bending and upsetting forces to the pipe.
When the pipe is bent with upsetting, the bending force is applied to the other end of the pipe, acting on the appropriate end of the pipe by means of hydraulic cylinders which feed said pipe longitudinally towards the bending head.
The pipe zone to be bent is heated by a circular heater, e.g. a H.F. inductor secured immovably to the bed of the device in the pipe-bending zone.
In view of the fact that bending of the pipe commences only after the heated zone reaches the temperature required for bending, the pipe and the heater remain for a certain period immovable relative to each other unitl bending begins. It has been proved in practice that a part of the heated zone located just behind the heater (looking from the bending head) becomes overheated within this period of time. This leads to uneven deformation of the pipe walls along the width of the heated zone and, as a consequence, to variations in the thickness and strength of the pipe manifested by corrugations or wrinkles in this part of the heated zone.
To counter the above-mentioned disadvantages, in recent times there has been a suggestion to bend the pipe with the aid of the same devices that are described above, but to place the pipe on the bed of the device so that in the initial position said pipe comes short of the stop in the bending head and remains unclamped in it until the actual process of bending begins. When the heater is switched on, the pipe is simultaneously loaded by a force insuring a free feed of the pipe to the stop of the bending head and, consequently, movement of the pipe relative to the heater. The period of time within which the front end of the pipe moves to the stop of the bending head is sufficient for stabilizing the temperature and width of the heated zone. Then the bending force is applied to the pipe (see, for example, Pat. No. 1,935,100, Cl.N21d-9-18, Federal Republic of Germany).
This method involves considerable difficulties consisting in the need for the operator to determine the moment when the front end of the pipe comes to the stop of the bending head in order to be able to clamp in due time the corresponding end of the pipe in the bending head.
This moment corresponds to the instantaneous stopping of the longitudinal movement of the pipe which precedes bending.
In addition to the difficulties in determining the moment of clamping the pipe in the bending head, a dangerous situation may arise because it becomes impossible to close the cover of the bending head if the moment of it clamping is missed.
Furthermore, clamping of the pipes under these conditions always results in considerable damage, i.e. deep dents produced on the pipe walls by the bending head clamps because the pipe in this method is always more or less cocked in the bending head just before clamping.